Rotation Barrier in a Doubly Vinylogous Amide

In the course of a synthesis of nicotinaldehyde, we followed the method of Arnold.¹This involves a bis-Vil-smeier-Haack formylation of aminoenamine 1, which is readily obtained from crotonaldehyde. The product of this reaction, an orange crystalline material with a violet luster and a sharp melting point, was described by Arnold as having structure 2, but alternative 3 was not excluded.¹On the basis of NMR spectroscopy, we can establish 3 as the correct structure for this substance, but in addition, we notice broad lines characteristic of an ongoing dynamic process. In this paper we determine the nature and the energy barrier of this process.